An Auditory Brainstem Implant (ABI) consists of two components, the external audio processor and the implant. The audio processor is either an OPUS 2 or OPUS 1.
The implant consists of an electronics housing and an implantable soft silicone matrix with a 12 contact electrode array. The 5.5 × 3.0 mm matrix is the active interface between the stimulation electronics and the neural tissue. An additional reference electrode is used for advanced telemetry measurements providing added functional reliability and control. The soft silicone matrix with a 12 contact electrode array is surgically inserted directly onto the brainstem. The implant stimulates the cochlear nucleus, which enables the recipient to distinguish a variety of sounds. Following surgery and the post-operative healing period of 14 days, the user is required to undergo an intensive training period with a qualified audiologist to learn how to interpret sounds and understand speech. Generally, it is important to realise that speech understanding with an ABI is not on the same level as speech understanding for cochlear implant recipients. Extensive training is all the more crucial for this reason, allowing the recipient to gain the best benefit from an ABI and to make the most of this advanced technology.
Patient selection is based on disease course, clinical signs, audiological, radiological and psycho-social criteria. The ABI provides the patients with access to auditory information such as environmental sound awareness together with distinct hearing cues in speech. In addition, this device markedly improves speech reception in combination with lip-reading. Nonetheless, there is only limited open-set speech understanding. Results of hearing function are correlated with electrode design, number of activated electrodes, speech processing strategies, duration of pre-existing deafness and extent of brainstem deformation. Functional neurostimulation of the central auditory system by a brainstem implant is a safe and beneficial procedure, which may considerably improve the quality of life in patients suffering from deafness due to bilateral retrocochlear lesions. The auditory outcome may be improved by a new generation of microelectrodes capable of penetrating the surface of the brainstem to access more directly the auditory neurons 1).
Worldwide till 2005, about 300 patients have already received an ABI through a translabyrinthine approach or suboccipital approach supported by multimodality intraoperative neurophysiological monitoring.
Auditory brainstem implants have failed to produce consistent clinical results comparable to those with the cochlear implant, both with surface and penetrating electrodes.
The MED-EL Auditory Brainstem Implant (ABI) is a solution for individuals with hearing loss due to a non-functioning auditory nerve (Neurofibromatosis Type 2). Bypassing both the inner ear and the auditory nerve, the MED-EL ABI stimulates the cochlear nucleus (CN) and provides users with a variety of hearing sensations to assist with sound awareness and communication.
ABI Candidacy An ABI is a solution for patients who are at least 15 years of age and who have been diagnosed with Neurofibromatosis Type 2 (NF2). With NF2, both cochlear nerves may no longer be able to function, or it may be anticipated that the nerves will no longer be able to function due to the presence or removal of a tumour. An ABI is therefore suitable for patients whose auditory nerves have been irreversibly damaged.
Attention: Device implantation and tumour removal shall both take place within the same surgery.
MED-EL ABI System Advantages Implantable soft silicone matrix with an active 12 point electrode array Comprehensive telemetry measurements pre and post surgery 12 individual stimulation contacts with separate current sources Highly sophisticated ABI surgical placement system
A multidatabase search identified three studies with individual patient data of longitudinal hearing outcomes after ABI insertion in adults. Data was collected on hearing outcomes of different sound complexities from sound to speech using an ABI ± lip reading ability plus demographic data. Due to heterogeneity each study was analyzed separately using random effect multi-level mixed linear modelling.
Across all three studies (N=111 total) there were significant improvements in hearing over time from ABI placement (p<0.000 in all). Improvements in comprehension of sounds, words, sentences, and speech occurred over time with ABI use plus lip reading but lip reading ability did not improve over time. All categories of hearing complexity had over 50% comprehension after over one year of ABI use and some subsets had over 75% or near 100% comprehension. Vowel comprehension was greater than consonant, while word comprehension was greater than for sentences (p<0.0001 in both). Age and gender did not predict outcomes.
ABIs improve hearing beyond lip reading alone, which represents baseline patient function prior to treatment, and the benefits continue to improve with time. These findings may be used to guide patient counseling regarding ABI insertion, rehabilitation course after insertion, and future studies 2).
There were 50 primary ABI insertions in 49 patients, including 16 inserted at the time of first side tumor removal as a sleeper, and two revision repositionings which failed to improve outcome. Postoperatively three patients had cerebrospinal fluid leaks which did not require reoperation, one patient had meningitis, and eleven patients suffered either temporary or permanent lower cranial nerve dysfunction. Twenty-nine patients became full time users; a further 12 patients became non-users. Three patients died while their device was inactive. Five patients retain serviceable contralateral hearing. Audiological open set testing of users showed means of: environmental sounds discrimination 51%; phoneme discrimination: with ABI alone 22%/lip reading (LR) 45%/ABI with LR 65%; sentence testing: with ABI alone 13%/LR 19%/ABI with LR 54%.
The majority of patients with NF2 implanted with an ABI find the device a useful aid to communication in conjunction with LR and in recognizing common environmental sounds. A small proportion gain open set discrimination. Almost a third of patients may end up as non-users. There is probably an increased risk of postoperative lower cranial nerve dysfunction so careful preoperative assessment is advised 3).
One hundred twelve ABI users were contacted and 43 patients (38.3)% responded to a survey. Tinnitus was reported in 83.7% of patients. The THI score for responders was 17.8 ± 20.5 standard deviation (SD). For survey participants, the ABI reduced tinnitus levels (mean VAS: Off = 3.5; On 1-h = 2.1; p = 0.048). For patients who subjectively reported that the ABI reduced tinnitus loudness, tinnitus levels were immediately reduced on ABI activation and after 1 hour of use (mean VAS: Off = 4.8; On = 2.4; On 1-h = 1.8; p < 0.01). Suppression did not continue after the device was turned off. Audiological performance with the ABI did not correlate with tinnitus suppression.
NF2 patients who have undergone removal of VS have a significant tinnitus handicap and benefit from tinnitus suppression through utilization of an ABI possibly through masking or electrical stimulation of the auditory brainstem 4).